A receiver, also known as User Equipment (UE), mobile station, wireless terminal and/or mobile terminal is enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made, e.g., between two receivers, between a receiver and a wire connected telephone and/or between a receiver and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The receiver may further be referred to as mobile telephones, cellular telephones, computer tablets or laptops with wireless capability. The UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another receiver or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, or base station, e.g., a Radio Base Station (RBS), which in some networks may be referred to as transmitter, “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. Sometimes, also the expression cell may be used for denoting the radio network node itself. However, the cell is also, or in normal terminology, the geographical area where radio coverage is provided by the radio network node/base station at a base station site. One radio network node, situated on the base station site, may serve one or several cells. The radio network nodes communicate over the air interface operating on radio frequencies with the receivers within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC), e.g., in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC), e.g., in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), radio network nodes, which may be referred to as eNodeBs or eNBs, may be connected to a gateway, e.g., a radio access gateway, to one or more core networks.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the radio network node to the receiver. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction, i.e., from the receiver to the radio network node.
During the initial access of a receiver/mobile terminal in a wireless communication system, a connection is established by first detecting and synchronizing to the cell. In the prior art 3GPP LTE system, the receiver establishes a connection to a cell/radio network node, by performing cell search and synchronization using primary and secondary synchronization signals. Once the cell ID has been found and synchronization been established by the receiver, a Physical Broadcast Channel (PBCH) is detected. The PBCH comprises a minimum amount of information for the receiver to be able to proceed receiving other data channels and start camping on the cell. The PBCH has been designed to be very robust since it needs to provide large coverage.
In LTE, the smallest time-frequency entity that can be used for transmission is referred to as a Resource Element (RE), which may convey a complex-valued modulation symbol on a subcarrier. In this context, the RE may be referred to as time-frequency resources. A Resource Block (RB) comprises a set of resource elements or a set of time-frequency resources and is of 0.5 ms duration (e.g., 7 Orthogonal Frequency-Division Multiplexing (OFDM) symbols) and 180 kHz bandwidth (e.g., 12 subcarriers with 15 kHz spacing). The LTE standard refers to a Physical Resource Block (PRB) as a resource block where the set of OFDM symbols in the time-domain and the set of subcarriers in the frequency domain are contiguous. The LTE standard further defines Virtual Resource Blocks (VRBs) which can be of either localized or distributed type. For brevity, sometimes only the notion of resource block is used and a skilled reader would be able to determine the proper term. The transmission bandwidth of the system is divided into a set of resource blocks. Typical LTE carrier bandwidths correspond to 6, 15, 25, 50, 75 and 100 resource blocks. Each transmission of user data on the Physical Downlink Shared Channel (PDSCH) is performed over 1 ms duration, which is also referred to as a subframe, on one or several resource blocks. A radio frame consists of 10 subframes, or alternatively 20 slots of 0.5 ms length (enumerated from 0 to 19).
OFDM is a method of encoding digital data on multiple carrier frequencies. OFDM is a Frequency-Division Multiplexing (FDM) scheme used as a digital multi-carrier modulation method. A large number of closely spaced orthogonal sub-carrier signals are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier.
OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, Digital Subscriber Line (DSL) broadband internet access, wireless networks, and 4G mobile communications.
In the prior art 3GPP LTE system, multiple transmit and receive antennas are supported and the notion of antenna port is used. Each downlink antenna port is associated with a unique reference signal. An antenna port may not necessarily correspond to a physical antenna and one antenna port may be associated with more than one physical antenna. In any case, the reference signal on an antenna port may be used for channel estimation for data that is transmitted on the same antenna port.
Channel estimation therefore needs to be performed for all antenna ports that are used for the data transmission. A number of reference signals have been defined in the LTE downlink, e.g., Common Reference Signal (CRS). CRS is a cell-specific reference signal, which is transmitted in all subframes and in all resource blocks of the carrier. The CRS serves as a reference signal for several purposes such as, e.g., demodulation, Channel state information measurements, Time- and frequency synchronization, and/or Radio Resource Management (RRM) and/or mobility measurements.
Up to 4 CRS antenna ports may be accommodated (labelled p=0-3) and a cell may be configured with ports p=0 or p=0, 1 or p=0, 1, 2, 3. Since the CRS provides for multiple purposes, it has to be rather robust and hence has quite a large density, i.e., occupies a large number of resource elements. The disadvantage is that its overhead is significant.
With multiple antennas, it may be at least hypothetically possible to achieve beamforming by applying different complex-valued precoder weights on the different antennas. However, since the CRS is cell-specific, it cannot be receiver-specifically precoded, i.e., it cannot achieve any beamforming gains. On the other hand, the user data on the Physical Downlink Shared Channel (PDSCH) may undergo beamforming since it is not cell-specific. This is done by codebook-based precoding and the chosen precoding matrix is signalled to the receiver. Since the CRS is not precoded and is transmitted every subframe, it is possible to interpolate channel estimates over both time- and frequency domain. This results in improved channel estimation.
A different approach for reference signal design was subsequently introduced in the LTE system, comprising two new reference signals, each having a specific purpose.
One such downlink reference signal defined in LTE is Channel State Information Reference Signal (CSI-RS). CSI-RS is a sparse receiver-specific reference signal used primarily for estimating Channel State Information (CSI) such as, e.g., Channel Quality Indicator (CQI), Pre-coding Matrix Indicator (PMI), Rank Indicator (RI), which the receiver reports to the transmitter/eNodeB. The CSI-RS is transmitted in all resource blocks of the carrier but with a configurable period in time and it is much sparser than the CRS. Up to 8 CSI-RS antenna ports may be accommodated.
Yet another downlink reference signal defined in LTE is Demodulation Reference Signal (DM-RS). DM-RS is a receiver-specific reference signal used primarily as phase and amplitude reference for coherent demodulation, i.e., to be used in channel estimation. It is only transmitted in the resource blocks and subframes where the receiver has been scheduled data, i.e., containing the PDSCH. Up to 8 DM-RS antenna ports may be accommodated.
The DM-RS time-frequency patterns for LTE are defined in the Technical Specification: 3GPP TS36.211 (retrievable over the Internet from: http://www.3gpp.org).
The antenna ports (labelled p=7-14) are multiplexed both by disjoint sets of time-frequency resources as well as by orthogonal cover codes within a set of same time-frequency resources. Since it is receiver-specific, the DM-RS may be precoded with the same precoder used for the PDSCH, hence beamforming gains may be achieved for the reference signal. Since the data and reference signal use the same precoder, the precoding becomes transparent to the receiver which may regard the precoder as part of the channel. Hence, the precoder is not signalled to the receiver. Typically, to maximize the throughput, different precoders may be used in different resource blocks, implying that channel estimates cannot be interpolated between resource blocks. However, the radio network node/eNodeB may signal to the receiver that the same precoder is assumed on a set of contiguous resource blocks, which would allow interpolation in the frequency domain, also being referred to as Physical Resource Block (PRB) bundling. The prior art LTE system does not support interpolation in the time domain, since the DM-RS is only transmitted in the subframes wherein data is transmitted and such transmission may not occur in each subframe, while also different precoders may be used in different subframes. Additionally, DM-RS is also utilised for demodulating some of the downlink control channels, e.g., the Enhanced Physical Downlink Control Channel (EPDCCH). These DM-RSs utilise the same time-frequency patterns as the DM-RSs for the PDSCH but may use another modulation sequence. The 4 EPDCCH antenna ports comprising DM-RS are labelled p=107-110.
FIG. 1 illustrates a resource grid in a subframe, which comprises 12 subcarriers and 14 OFDM symbols, to which antenna ports p=0-3 comprising CRS and antenna ports p=7-14 comprising DM-RS are mapped. The frequency position of the CRS depends on a frequency shift which is a function of the cell ID. FIG. 1 also shows the resource elements for the PBCH.
In the prior art LTE system, at least antenna port p=0 is always transmitted, regardless of the number of configured antenna ports comprising CSI-RS and antenna ports comprising DM-RS. To reduce the overhead, it has been considered to define carriers which do not transmit any antenna port comprising CRS, i.e., antenna ports p=0, 1, 2 and 3. However, there may still be a reference signal which is similar to CRS, e.g., using the same time-frequency resources and/or modulation sequence but is not transmitted in each subframe (e.g., only transmitted in some but not all subframes) and whose purpose is not to serve as a reference for demodulation. Rather, it could be used for measurements related to radio resource management and cell selection procedures. However, the PBCH is relying on CRS-based demodulation since it is transmitted on the antenna ports comprising CRS. Hence, the PBCH as it is currently defined in the prior art LTE system, could not be transmitted. Without the PBCH it would not be possible to access the carrier and it is therefore an open issue to provide a mechanism for transmitting broadcast information and an associated reference signal on a carrier without CRS.
According to prior art, the PBCH in LTE is transmitted in the 6 central resource blocks (72 subcarriers) of the carrier and in the first 4 OFDM symbols of slot 1. The smallest LTE transmission bandwidth configuration of a carrier is 6 resource blocks and the receiver does not know the carrier bandwidth prior to detecting the PBCH. Using 6 resource blocks assures that the PBCH may be detected regardless of the carrier bandwidth and at the same time it provides maximum frequency diversity. FIG. 1 illustrates the PBCH mapping for FDD in one Physical Resource Block (PRB) pair. The data of the PBCH is convolutionally encoded and a 16-bit Cyclic Redundancy Check (CRC) is attached to provide for error detection. The transmission time of the PBCH is 40 ms, i.e., the encoded data is conveyed over 4 radio frames, using the first 4 OFDM symbols of slot 1 in each radio frame. However, the information is mapped such that it would be possible to correctly receive the PBCH from just 1 decoding attempt, i.e., from 1 radio frame. On the other hand, the 40 ms timing is unknown to the receiver which needs to be detected. The scrambling sequence of the PBCH is defined over 40 ms, hence the receiver can blindly detect the 40 ms timing, even from 1 decoding attempt, requiring 4 decoding hypotheses. Having a transmission time of 40 ms spreads the broadcast message over several radio frames and assures that time-diversity can be achieved, e.g., in order to avoid fading dips.
The PBCH is transmitted on the antenna ports comprising CRS. The number of CRS antenna ports may be 1, 2 or 4 but this number is unknown to the receiver prior to detecting the PBCH. Transmit diversity is used for the PBCH when there is more than 1 antenna port comprising CRS. For 2 antenna ports comprising CRS, Space Frequency Block Coding (SFBC) is applied and for 4 antenna ports comprising CRS a combination of SFBC and Frequency Switched Transmit Diversity (FSTD) is used. The receiver is blindly detecting the number of antenna ports comprising CRS by de-mapping the REs of the PBCH under the 3 hypotheses of 1, 2 or 4 antenna ports comprising CRS and corresponding diversity scheme. The PBCH is always mapped to the resource elements assuming 4 antenna ports comprising CRS are used. That is, the resource elements defined for antenna ports p=0-3 are never used to carry the PBCH, regardless of the number of actually configured antenna ports. The CRC is masked with a codeword dependent on the number of antenna ports comprising CRS. Hence, the receiver may verify if the correct number of antenna ports comprising CRS has been detected.
As illustrated in FIG. 1, multiplexing of DM-RS antenna ports is made both by Code Division Multiplexing (by means of orthogonal cover codes) and with Frequency Division Multiplexing, by means of disjoint sets of resource elements. For example, antenna ports p=7, 8, 11 and 13 would be multiplexed by orthogonal cover sequences (the same would apply for antenna ports p=9, 10, 12 and 14). A rank v transmission on the PDSCH is using antenna ports 7 to v+6, at least when the rank v>1. The modulation sequence of the DM-RS can be receiver-specifically configured, i.e., it may not be the same for all receivers in the cell. The reason for this is that it makes it possible to reuse time-frequency resources within a cell for receivers that are sufficiently spatially separated. Having different DM-RS modulation sequences improves the interference suppression ability of the reference signals within the cell. There is no transmit diversity scheme specified for channels that rely on DM-RS demodulation in the prior art LTE system. Still there are transparent ways of improving the spatial diversity, e.g., to apply different precoders in different resource blocks, which sometimes may be referred to as Random Beamforming (RBF). However, in many cases the performance may be worse than schemes relying on transmit diversity, e.g., SFBC.
It is a problem to reliably transmit a broadcast information and/or system information while offering high spectral efficiency of a wireless communication system. This necessitates the ability to multiplex the broadcast information with other channels in the system, including the mapping of the broadcast information to time-frequency resources and definitions of reference signals for its associated antenna ports. It is a further problem to define the reference signals for broadcast information transmission such that the complexity of channel estimation does not become excessive in the receiver. It is also a problem to define the reference signals for broadcast information transmission such that the complexity of mapping other data/control channels to the time-frequency resources is not increased.
In the prior art LTE system, both CRS and DM-RS can be transmitted, which leads to high reference signal overhead, decreased throughput and reduced overall system efficiency. It is a further objective to define the reference signals for broadcast information transmission in order to minimize the overhead.
Hence, it is a problem to assure that there is a reasonable trade-off between reference signal overhead and performance.
In the sequel, we will refer to methods for transmitting broadcast information in a general sense. This mechanism may alternatively be referred to, in some specific cases, as a broadcast channel. Hence, these terms may be used interchangeably without precluding that a method for transmitting broadcast information may not necessarily require defining a specific broadcast channel.